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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xmlns="http://docbook.org/ns/docbook" xml:id="function.crc32">
<refnamediv>
<refname>crc32</refname>
<refpurpose>Calculates the crc32 polynomial of a string</refpurpose>
</refnamediv>
<refsect1 role="description">
&reftitle.description;
<methodsynopsis>
<type>int</type><methodname>crc32</methodname>
<methodparam><type>string</type><parameter>string</parameter></methodparam>
</methodsynopsis>
<para>
Generates the cyclic redundancy checksum polynomial of 32-bit
lengths of the <parameter>string</parameter>. This is usually used
to validate the integrity of data being transmitted.
</para>
<warning>
<simpara>
Because PHP's integer type is signed many crc32 checksums will
result in negative integers on 32bit platforms. On 64bit installations
all <function>crc32</function> results will be positive integers though.
</simpara>
<simpara>
So you need to use the "%u" formatter of <function>sprintf</function> or
<function>printf</function> to get the string representation of the
unsigned <function>crc32</function> checksum in decimal format.
</simpara>
<simpara>
For a hexadecimal representation of the checksum you can either use the
"%x" formatter of <function>sprintf</function> or <function>printf</function>
or the <function>dechex</function> conversion functions, both of these
also take care of converting the <function>crc32</function> result to
an unsigned integer.
</simpara>
<simpara>
Having 64bit installations also return negative integers for higher
result values was considered but would break the hexadecimal conversion
as negatives would get an extra 0xFFFFFFFF######## offset then. As
hexadecimal representation seems to be the most common use case we
decided to not break this even if it breaks direct decimal comparisons
in about 50% of the cases when moving from 32 to 64bits.
</simpara>
<simpara>
In retrospect having the function return an integer maybe wasn't the
best idea and returning a hex string representation right away (as
e.g. <function>md5</function> does) might have been a better plan to
begin with.
</simpara>
<simpara>
For a more portable solution you may also consider the generic
<function>hash</function>. <code>hash("crc32b", $str)</code> will
return the same string as <code>str_pad(dechex(crc32($str)), 8, '0', STR_PAD_LEFT)</code>.
</simpara>
</warning>
</refsect1>
<refsect1 role="parameters">
&reftitle.parameters;
<para>
<variablelist>
<varlistentry>
<term><parameter>string</parameter></term>
<listitem>
<para>
The data.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect1>
<refsect1 role="returnvalues">
&reftitle.returnvalues;
<para>
Returns the crc32 checksum of <parameter>string</parameter> as an integer.
</para>
</refsect1>
<refsect1 role="examples">
&reftitle.examples;
<para>
<example>
<title>Displaying a crc32 checksum</title>
<para>
This example shows how to print a converted checksum with the
<function>printf</function> function:
</para>
<programlisting role="php">
<![CDATA[
<?php
$checksum = crc32("The quick brown fox jumped over the lazy dog.");
printf("%u\n", $checksum);
?>
]]>
</programlisting>
</example>
</para>
</refsect1>
<refsect1 role="seealso">
&reftitle.seealso;
<para>
<simplelist>
<member><function>hash</function></member>
<member><function>md5</function></member>
<member><function>sha1</function></member>
</simplelist>
</para>
</refsect1>
</refentry>
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